Improve vcs_branch printing for git
Improve vcs_branch printing for git

If browsing a git repository in detached head mode, the branch name used
to say "(No branch)". This is not helpful. Now, it prints a colon and
the abbreviated sha-name that represents the commit you are currently
browsing (i.e. ":303d0sd")

tacahiroy/tmux-powerline,tacahiroy/tmux-powerline,tacahiroy/tmux-powerline